Ground Fault Interrupters

A groundfault circuit interrupter (GFCI) is an electrical device which protects personnel by detecting potentially hazardous ground faults and quickly disconnecting power from the circuit. A potentially dangerous ground fault is any amount of current above the level that may deliver a dangerous shock.

Ground fault circuit interrupters (GFCI) are found in newer homes, usually in the kitchen, bathroom, laundry room and garage. They are small circuit breaker-like devices which shut off the associated circuit if there is an electrical "leak" which may cause a "ground fault" - a shock hazard due to moisture or a damp floor. Sometimes a GFCI is installed on the main breaker box, thereby protecting the entire circuit from ground faults.

A GFCI works by comparing the amount of electrical current coming into a circuit (on the black wire) with the amount leaving ( on the neutral or red wire). If more current enters the circuit through phase wire than leaves through neutral wire, there is a current leak or ground fault. GFCI is able to detect as little as a 0.005 amp leak, a GFCI can shut down a circuit within 0.025 seconds, helping prevent serious electrical shocks. A fault current as low as 4 mA to 6 mA activates the GFCI and interrupts the circuit. Once activated, the fault condition is cleared and the GFCI manually resets before power may be restored to the circuit.

Residual Current Devices (like GFCI) pass live and neutral currents in opposition through two windings on the same toroidal core. A third winding is energised if there is any unbalanced flux, due to live current flowing directly to ground through a fault or a body, and this winding is arranged to trip a circuit breaker.

GFCI protection may be installed at different locations within a circuit. Direct-wired GFCI receptacles provide a ground fault protection at the point of installation. GFCI receptacles may also be connected to provide GFCI protection at all other receptacles installed downstream on the same circuit. GFCI CBs, when installed in a load center or panelboard, provide GFCI protection and conventional circuit overcurrent protection for all branch-circuit components connected to the CB.

Plug-in GFCls provide ground fault protection for devices plugged into them. These plug-in devices are often used by personnel working with power tools in an area that does not include GFCI receptacles.

Seriously consider adding GFCI,s to any and all circuits that may have a ground hazard. Test GFCI's once a month by pushing the "test" button. The power should shut off in the circuit, indicating that the GFCI is working properly. Press the reset button to restore power.

The following article from sci.engr.electrical.compliance newsgroup is published in this web site with permission form Rich Nute.

From: (Rich Nute)
Newsgroups: sci.engr.electrical.cmpliance
Subject: Re: Ground Fault Interrupters - Internationally
Date: 18 Dec 1996 20:47:20 GMT
Organization: Hewlett-Packard, San Diego Division
Lines: 65
Distribution: world
Message-ID: <599l8o$>
References: <> <0CsvKVAIJYpyEwf4@m> <> <58tnov$> <MPG.d20bbd5> <>
Hello from San Diego:
GFCI (aka GFI)  =  Ground Fault Circuit Interrupter  (USA, Canada)
ELCB (aka ELB)  =  Earth Leakage Circuit Breaker     (U.K.)
RCCB (aka RCB)  =  Residual Current Circuit Breaker  (Europe)
All of these devices operate on the same principle.

The live and neutral wires are passed through a magnetic core. Under normal conditions, because the current in both conductors is equal and opposite, there is no flux in the core.

Under certain fault conditions, the current in the live wire can exceed the current in the neutral wire. This produces flux in the core.

In the GFCI, the core has a secondary winding. When there is flux in the core, the secondary winding produces a voltage. The voltage is applied to a solid-state amplifier whose output is applied to a relay coil. The relay interrupts the line and neutral conductors.

Typical GFCI is built into a 120-volt duplex outlet. It trips at 5 mA.

Other GFCIs are built into 15-amp circuit breakers. These trip at 20 mA to account for the leakage of the wiring between the panel board and the devices being protected.

GFCIs almost always include a test button and a reset button.

In ELCBs and RCCBs, the core is part of a relay. The magnetic flux is coupled directly to the relay, which opens the circuit. Because these devices are totally magnetic, they require relatively high flux to operate. So, typically, ELCBs and RCCBs operate at 20 mA or more.

The typical ELCB/RCCB is built into a 230-volt circuit breaker and is located in the panel board.

I've never seen a test button on an ELCB/RCCB. Instead, they include a reset lever for resetting after either a ELCB/RCCB- caused trip or an overcurrent-caused trip.

In the USA, many professionals believe that the GFCI detects current in the ground wire. GFCI manufacturers support this myth by requiring the GFCI be only installed where a ground wire can be connected.

Because the GFCI/ELCB/RCCB operates on differential current, a ground wire is not essential to its operation. A GFCI operates equally well without a ground wire connected to it. I've done it! (This is not a recommendation or advice to install a GFCI contrary to manufacturers instructions; rather, I simply want to make the point that GFCIs operate on differential current, not on ground wire current.)

Richard Nute
San Diego